When devices, such as packaged integrated circuits (or “chips”) are built, it is conventional to stress test either each device or a sample of the devices that are built before shipping the devices to the customer.
One type of device is a programmable system on a chip device, for example, a PSoC® device manufactured by Cypress Semiconductor Corporation. A majority of internal logic components of a programmable system on a chip device are protected from a high-voltage supply by internal voltage regulators. The internal voltage regulators allow circuit designers to build the logic circuits with low-voltage transistors, as opposed to costly high-voltage transistors (also referred to as high-voltage switches). However, currently there are no external mechanisms to stress test these internal voltage regulated areas of the chip. Circuits that do not have internal voltage regulators can be run at higher voltages to determine problems with the circuit. However, areas of a chip that are regulated by an internal voltage regulator can not be run at higher voltages because the internal voltage regulator is designed to protect the circuitry from higher voltages.
Design for testing (DFT) is one means for chip manufacturers to supplement or supplant traditional functional testing role in which chips are tested at their input/outputs (I/O) for functional performance. The tests generally are driven by test programs that execute in automatic test equipment or inside the assembled chip itself. Conventional DFT methodologies include on-chip testing of device sub-blocks to indicate the presence of defects (i.e., the test fails) of circuitry within a chip. However, currently there are no DFT circuits to stress test areas of the chip (e.g., internal logic components) that are protected from the high-voltage supply by internal voltage regulators.
Conventional internal voltage regulators can include a high-voltage transistor, such as a regulator FET. High voltages are usually measured by dividing the high voltage down for comparison with lower voltage references. Resistive or capacitive dividers are commonly used. If the high voltage to be measured is also high impedance, then resistive dividers must use large valued resistors which are expensive in terms of die area. Capacitive dividers can perform the same function in less die area, but require high-voltage switches. High-voltage switches commonly require high-voltage control signals. The generation of these high-voltage control signals usually involves creating a high-voltage power supply with a charge pump and using level shifters to shift logic level control signals up to high voltages.